ANSAN, South Korea/SEOUL, April 25 (Reuters) - A floral tribute to the children who drowned in a sinking South Korean ferry features photographs of the victims in their school uniforms and lines of empty spaces waiting to be filled with photos as the confirmed death toll rises.

The pictures, flowers and spaces are banked up the entire wall of a gymnasium near their school on the outskirts of Seoul.

"There are too many pictures, way more than I thought," said crying university student Jung Sun-a, 24. "And they are too young in these pictures. I really hope they can fulfil their dream in the next life. And I hope the missing kids will also come back to their parents as soon as possible."

One wailing old woman shouted out for her granddaughter, Bomi.

"Bomi is still in darkness. She hasn't come home yet. What are we going to do? I came here to ask you. She is still in dark waters. What am I supposed to do?"

The Sewol ferry, weighing almost 7,000 tons, sank on a routine trip from the port of Incheon, near Seoul, to the southern holiday island of Jeju. Investigations are focused on human error and mechanical failure.

More than 300 people, most of them students and teachers from the Danwon High School, in Ansan, have died or are missing and presumed dead after the April 16 disaster.

The confirmed death toll on Friday was 181.

Classes at the high school resumed on Thursday in sombre mood. Children filed past the floral tribute, offering white chrysanthemums. Yellow ribbons, with names and messages inscribed, were tied around a fence.

In the classrooms of the missing, friends posted messages on desks, blackboards and windows, in the days after disaster struck, asking for the safe return of their friends.

One note was stuck to the window of an empty classroom in the days when hopes for finding the passengers alive were fading fast. It was addressed to a girl called Si-yeon.

"Si-yeon, It's me, your oppa (male friend). I miss you a lot. I am so worried. Please come out of that ferry. We ought to go and eat your favourite things, sweet potatoes, cheese and tangerines," it read.

"It must be really cold in there. I am so sorry that I cannot do anything for you. It makes me feel so frustrated - there is nothing else I can do but pray for you. I don't even want to imagine how scared you must be. I hope you are alive. I won't give up, I will wait for you. I want to tell you that I love you."

CREW MEMBERS ARRESTED

Captain Lee Joon-seok, 69, and other crew members who abandoned ship after telling the children to stay put in their cabins have been arrested on negligence charges. Lee was also charged with undertaking an "excessive change of course without slowing down".

One crew member said after a brief appearance in court on Thursday she and six colleagues had been "under command" to abandon ship. Another was asked if there was any discussion about trying to save the children.

"At that moment, we were on the third floor and except for the third floor situation, we weren't aware of anything else," the crew member said.

Prosecutors said on Thursday they had raided two shipping watchdogs, the Korean Shipping Association and the Korean Register of Shipping, as part of their expanded investigation.

They also raided the home of Yoo Byung-un, the head of a family that owns the Chonghaejin Marine Co. Ltd, the company that operated the Sewol. They had also seized another ferry run by the company to check for safety.

The ship, 146 metres (479 ft) long and 22 metres wide, was more than three times overloaded, according to official recommendations, with cargo poorly stowed and inadequate ballast.

Moon Ki-han, an executive at Uryeon (Union Transport Co.), the firm that supervised cargo loading, told Reuters there were 105 containers onboard, some of which toppled into the sea as the ship listed.

Forty-five were loaded on to the front deck and 60 into the lower decks, Moon said. In total, the ship was carrying 3,600 metric tons of cargo including containers, vehicles and other goods, he said.

A member of parliament this week said the Korean Register of Shipping recommended a load of 987 tons for the Sewol.